Acousto-optical systems are known for providing an efficient way of influencing light by either filtering out light of one or more particular useful wavelengths or bands of wavelength, or deflecting the light, either selectively deflecting certain wavelengths or bands of wavelengths, or deflecting an entire light beam.
Acousto-optical elements are used in various manners, particularly in the field of microscopy. For instance, both in wide field microscopes and in laser scanning microscopes, it is desirable to provide a certain light comprising a mixture of certain wavelengths as incident light for illuminating an object to be imaged. For enhancing the quality of the image, it might be useful to adjust the mix of colors, i.e. wavelengths of light, in the incident light beam.
In the field of confocal scanning microscopy, it is of particular interest to adjust the intensities for certain wavelengths or to turn certain wavelengths on or off. Particularly in the field of fluorescence microscopy that might be either confocal microscopy or wide field microscopy, it is important to excite the dye with a particular wavelength for achieving fluorescent emission of light that is detected and used for creating the desired image of the object to be imaged. Several wavelengths are particularly needed if the object is dyed with dyes of different emission wavelengths for obtaining multi-color pictures.
In various types of microscopy, more than one wavelength is needed, for example in the field of stimulated emission depletion microscopy (STED) where the light of a first wavelength is used for excitation of fluorophores, while light of a second wavelength is used for depleting the excited states of the fluorophores in parts of the exciting spot for narrowing the effectively excited spot in order to obtain an image of a higher resolution. Other fields of microscopy using light of several discrete wavelengths are Raman microscopy, coherent anti-Stokes Raman microscopy (CARS) and SRS microscopy.
In summary, the various functions of filtering, deflecting, and beam splitting of broadband light or line spectrum light or of laser light of the discrete wavelengths are often performed in microscopes by acousto-optical elements. Examples for such acousto-optical elements are acousto-optical tunable filters (AOTF), acousto-optical modulators (AOM), acousto-optical deflectors (AOD), acousto-optical beam splitters (AOBS), and acousto-optical beam mergers (AOBM).
Of all the various acousto-optical elements that are used in the field of microscopy, acousto-optical tunable filters (AOTF) are the most commonly used, but also acousto-optical beam splitters (AOBS) comprising AOTFs are used in the field of microscopy. These types of acousto-optical elements influence the light of specific wavelengths in contrast to for example AOMs, AODs and Frequency Shifters that influence the entire light.
The basic structure of an acousto-optical element comprises a crystal and a transducer that is attached to the crystal. The transducer is configured to receive an electronic signal, typically in the radio frequency range between 30 Mhz and 800 Mhz. The transducer converts the electronic signal into an acoustic signal by physically contracting and expanding according to the electronic signal. The crystal oscillates physically according to the acoustic signal and therefore forms the optical equivalent of an optical diffraction grating deflecting selectively light of particular wavelengths. Particularly in an AOTF the properties of the crystal are such that each acoustic wavelength results in deflecting only a particular optical wavelength, or more specifically a narrow bandwidth of optical wavelengths, e.g. of about 3 nm, while only the exact wavelength that is correlating to the respective acoustic frequencies is deflected by 100 percent, while adjacent wavelengths within the narrow 3 nm band are deflected by a lower percentage, for instance only by 50 percent. The typical use of acousto-optical elements involves modifying an electronic driver signal driving the transducer and therefore modifying the acoustic signal generated by the transducer over time, mainly amplitude modulation for changing the strength of the signal over time and therefore for modulating the intensity of the deflected light of the various desired specific wavelengths over time.
For influencing several frequencies at the same time, the electronic signals in the radiofrequency range have to be combined or superposed into one signal if influencing several wavelengths by only one crystal and only one transducer is desired. The various radio frequencies are in the prior art generated by oscillating circuits like for instance voltage-controlled oscillators (VCO) wherein the oscillator frequency is controlled by a voltage input. Other types of oscillators that are typically used are phase-locked loop oscillators (PLL) or direct digital synthesizers (DDS). All of these frequency generators have in common that these generate an analog signal that is then as aforementioned typically amplitude modulated. For example, in AOTFs, the frequencies correlate to specific wavelengths, and for influencing each wavelength it requires a respective frequency generator, and the maximum number of wavelengths is determined by the number of frequency generators. Particularly in the field of fluorescence microscopy this is a significant limitation since expanding the use to additional excitation wavelengths requires providing a respective number of frequency generators. Another disadvantage is that non-linearity is difficult to compensate.
If several radiofrequencies are combined—not wavelengths—, particularly if analog electronic signals of several frequencies are superposed, i.e. combined into one combination signal, the maximum amplitude increases with the number of combined signals and results in a higher degree of non-linearity. Since the superposition increases the intensity, the system enters increasingly the range of nonlinearities. This is in many cases then seen as crosstalk because it seems to the user that the different radio frequencies influence each other. The result for the respective specific wavelengths is a lower acoustic signal generated by the transducer in comparison to using only one single wavelength. Put in other words, the more signals of different wavelengths are superposed, the more the signal strength for the individual wavelength decreases to some extent, resulting in a non-linear response of the system, i.e. the intensity of the generated acoustic waves is not a linear function of the strength of the electronic signal. Since all frequencies in the prior art are generated by individual frequency generators and then amplitude modulated by individual amplitude modulators, it is difficult to compensate for such cross-talk since this would require detecting information about the signal intensity by the other frequency generators in combination with their respective amplitude modulators. Even if such information is detected, this would require data processing and then feeding back the information to the respective individual amplitude modulator so that no “real time” compensation is possible, but only with a time delay resulting from detecting the signal intensity is from the other frequency generators in combination with their respective amplitude modulators and then data processing this information.
As a result, in the prior art, only signals of relatively low intensity are superposed, and both the amplifier and transducer are dimensioned adequately for operating these signals only in the linear range, i.e. are dimensioned to be relatively powerful and operated only at a small fraction of their capacity for staying within the linear range. This has not only significant cost disadvantages due to the higher price of these more powerful components, but also causes other technical difficulties like high structural dimensions, generating heat that needs to be dissipated, high-energy consumption, and the risk of damaging or even destroying the crystal of the acousto-optical element in case of an incident of inadvertent excessive amplification.
Generally, another possibility of keeping the maximum amplitude in case of several superposed signals at a lower level is to control the phases of the various signals with respect to each other prior to superposing these, i.e. preferably have each signal that is combined at a different phase than any or some of the other signals. Again, since the frequency generators and their respective amplitude modulators are individual elements in the prior art, this would require detecting the various phases and then adjusting these, which cannot be done in real-time and would require costly detecting and data processing units.
Another problem in the prior art is an excessive number of electronic elements, like a high number of frequency generators and amplitude modulators, particularly if flexibility for adding more frequencies for different uses of influencing light of different wavelengths is desirable.
It is an object of the invention to reduce the costs for acousto-optical systems that are capable of processing two or more signals.
It is further an object to allow more flexibility as to the various frequencies of the generated signals that are combined into one driver signal.
It is another object of the invention to reduce the energy consumption of the acousto-optical system.
It is another object of the invention to reduce the total number of electronic components.
It is another object of the invention to reduce the electronic components in size and capacity, further reducing costs and avoiding technical problems resulting from overdimensioned components.